Anabolic resistance to nutrition is the reduced ability of skeletal muscle to increase protein synthesis in response to feeding. It is a major contributor to muscle atrophy in aging, inactivity, burns, trauma, and cancer cachexia. The effects of anabolic resistance on health and physical function are important. For example, the loss of muscle mass and strength with aging (sarcopenia) increases the risk for falls, physical dependency and morbidity in older adults. A major determinant of muscle size is muscle protein content, which is controlled by the fine balance between protein synthesis and breakdown. Recently, we have found that amino acids and exercise independently increase muscle protein synthesis and overall anabolism by activating the mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway in humans. Aging and inactivity reduce these anabolic effects, but the underlying mechanisms of anabolic resistance are not known. The purpose of our application is to better understand how anabolic resistance develops in skeletal muscle. Our long-term goal is to identify specific molecular targets for the development of evidence-based clinical interventions to counteract anabolic resistance and muscle wasting in clinical populations. Here, we will focus on the three potential mechanisms underlying anabolic resistance to amino acids: a) endothelial function (i.e., amino acid-induced endothelium dependent vasodilation and consequent muscle perfusion); b) amino acid transport capacity and function; c) activation of mTORC1 in human muscle cells. Our central hypothesis is that the physical activity level regulates endothelial function which is the primary contributor to anabolic resistance in human skeletal muscle. We will test this hypothesis in healthy subjects with the following specific aims: 1) Determine how physical inactivity reduces the sensitivity of muscle to amino acids. 2) Determine how exercise improves the muscle sensitivity to amino acids. 3) Determine the effect of increasing habitual physical activity on anabolic resistance. We will study human subjects utilizing a novel 4-pool stables isotopic model to measure amino acid kinetics and muscle protein metabolism in combination with molecular analysis of muscle to determine the regulatory role of amino acids, physical inactivity, and amino acid transporter functional activity on mTORC1. The proposed approach is innovative because it represents a new and substantial departure from the status quo as we will examine the underlying mechanisms of anabolic resistance to nutrition using novel methodological approaches. The proposed research is significant because it will lead to the development of evidence-based interventions to treat sarcopenia and muscle wasting.